I've never sold only in pairs, but I have offered slight discounts for pairs and trios.
When my retics breed, I will definitely be listing them as 1.1 pairs and a discussion may persuade me to sell singles. Part of it is encouraging pure locality breeding, but part is discouraging SD males to mainland females. I can't prevent it if that's what people want to do, but I can at least make it harder for them to waste pure locale SDs on that. I was on a wait list for years before I got mine, and plenty of people just like me are patiently waiting because they want pure locale.